Studies on the therapeutic utility of 13-cis retinoic acid in the treatment of Fibrodysplasia Ossificans Progressiva (FOP) continues. Treatment of rats with 13-cis retinoic acid at 10 mg/kg daily, inhibited induction of heterotopic ossification in response to the implantation of demineralized bone matrix, suggesting the potential therapeutic utility of this vitamin A derivative in the treatment of FOP. Evaluation of a nine-month limited clinical trial with 13-cis retinoic acid in 8 patients with FOP suggests that this drug can significantly inhibit the formation of heterotopic cartilage and bone in these patients, without significant systemic toxicity.